Organic light-emitting devices (OLEDs) are finding applications as next-generation flat-panel displays (FPDs), liquid crystal display (LCDs), and plasma display panels (PDPs). This has been driven by their favorable properties including lightweight, fast video response and low power consumption. To this end, organometallic compounds exhibiting electroluminescence are particularly attractive for electrophosphorescent applications, since both the ligand structure and the central metal atom can be varied to modify the properties of the device using these compounds.
Both fluorescent emission and phosphorescent emission are utilized in OLED technology based on electroluminescence. In an electroluminescent device, light emission from a fluorescent emitter occurs as a result of relaxation of singlet excitons in the emissive layer. U.S. Pat. No. 6,310,360 discloses that such emission is theoretically limited to internal quantum efficiencies of 25%. In contrast, phosphorescent emission occurs as the result of a forbidden formation of excitons, for example, when a triplet spin state relaxes or decays to a singlet spin state.
In recent years, the application of phosphorescent emitters in OLEDs has received considerable attention. Through the heavy atom effect of a transition metal, light emission results from radiative formation of triplet excitons due to the efficient intersystem crossing from singlet to triplet excited states. This results in internal quantum efficiencies of up to 100% (see Baldo et al., Nature 395:151(1998); Adachi et al., Appl. Phys. Lett., 77:904 (2000)).
Electrophosphorescent materials with different color emissions are known. Thompson et al. at the University of Southern California and Forrest et al. at Princeton University jointly reported a family of iridium complexes exhibiting improved brightness and efficiencies (see, e.g., U.S. Pat. No. 6,515,298 B2; U.S. Patent Application Publication No. 20020182441 A1; Lamansky et al., J. Am. Chem. Soc., 123:4304 (2001); and Xie et al., Adv. Mat., 13:1245 (2001)). Che and co-workers have demonstrated the use of metal organic complexes, such as platinum (II), copper (I), gold(I), and zinc(II) complexes, as OLED materials (see Y.-Y. Lin et al., Chem. Eur. J., 9:1263 (2003); Lu et al., Chem. Commun., 206 (2002); Ma et al., New J. Chem., 263 (1999); Ma et al., Appl. Phys. Lett., 74:1361 (1999); Ho et al., Chem. Commun., 2101 (1998); and Ma et al., Chem. Commun., 2491 (1998)). U.S. Pat. No. 6,048,630 discloses an electroluminescent device, based on phosphorescent Pt(OEP)(H2OEP=octylethylporphyrin), which emits a saturated red color. In addition, international patent application No. WO 00/57676 discloses the use of electrophosphorescent dopants such as cis-bis[2-(2′-thienyl)pyridinato-N,C3]platinum (II) (Pt(thpy)2). Lamansky et al., Organic Electronics 2:53 (2001) disclose polymer-based OLEDs using platinum 2,8,12,17-tetraethyl-3,7,13,18-tetramethyl porphyrin (PtOX).
The use of organometallic compounds in OLEDs is also known. Adachi et al., Appl. Phys. Lett., 77:904 (2000) discloses an OLED generating red emissions with internal quantum efficiencies of 23% comprising 2,3,7,8,12,13,17,18-ocatethyl-21H,23H-porphine platinum(II) as the dopant. U.S. Pat. No. 6,048,630 discloses an OLED emitting a saturated red color comprising Pt(octylethylporphyrin) and a receiving compound.
In addition, cyclometallated complexes, where the metal is chelated to a nitrogen-heterocycle via both a N atom and a C atom are also reported to be useful in making OLEDs. U.S. Pat. No. 6,515,298 B2 discloses an electrophosphorescent device comprising an emissive layer comprising cyclometallated tris(2-phenylpyridine)metal compounds and an intersystem crossing molecule, where the metal is bonded to at least one carbon atom of the ligand.
U.S. Patent Application Publication No. 20020182441 A1 discloses OLEDs comprising cyclometallated iridium complexes in the emissive layer. The disclosed complexes have a metal atom bound to at least one mono-anionic, bidentate, carbon-coordination ligand and at least one non-mono-anionic, bidentate, nitrogen-coordination ligand. According to the reference, the resulting devices emit light in the blue, green, or red region of the visible spectrum, with the emission exhibiting a well-defined vibronic structure. Synthesis of such assymmetric complexes is necessarily more complex than the synthesis of symmetric organometallic complexes even though the organic ligands themselves may be assymmetric.
Lamansky et al. J. Am. Chem. Soc., 123:4304 (2001) discloses OLEDs with a cyclometallated iridium(acetylacetenato) complex in the emissive layer that provides an OLED exhibiting green, yellow and red electroluminescence, wherein the emission of the device can be changed by varying the structure of the nitrogen-heterocycle.
Besides being an alternative to a conventional illumination source, white organic light-emitting devices (WOLEDs) are expected to be useful in full color flat-panel display technology. J. Kido et al. suggested using WOLED arrays in which the white-light emission can be converted to red, green and blue (R-G-B) colors by color filters in a facile approach for the development of full color OLED display (see Kido et al., Science 267:1332, (1995)). By stacking emissive layers, white-light emission in OLEDs can be achieved. (See Andrade et al., Adv. Mater., 14:147 (2002); Huang et al., Appl. Phys. Lett., 80:2782 (2002); and Ko et al., Appl. Phys. Lett., 79:4234 (2001)).
WOLEDs have also been implemented with exciplexes being the emitting materials to take advantage of the broad spectrum produced by such materials. Exciplex based devices exhibit low quantum efficiencies, which typically do not exceed 0.6 lm/W. As consequence, typically, WOLEDs utilize multiple R-G-B dyes to provide the broad visible spectrum. (See international publication No. WO 02/091814 A2; U.S. Patent Application Publication No. 20020197511 A1; Kawamura et al., J. Appl. Phys., 92:87 (2002); Duggal et al., Appl. Phys. Lett., 80:3470 (2002); and Ko et al., Appl. Phys. Lett., 79:4234 (2001)). As may be expected, the use of multiple dyes in emission layers requires fine adjustment of the concentration of each dye.
Adamovich et al. in New. J. Chem., 26:1171 (2002) disclose WOLEDs containing the single dopant, platinum (II) [2-4,6-difluorophenyl]pyridinato-N,C2′]β-diketonate in the emissive layer where an electron/exciton blocking layer is sandwiched between a hole transporting layer and the emissive layer. According to this reference, this sandwich structure improves the efficiency and color stability of the device. Moreover, Adamovich et al. reported that aggregation of such dopants results in generation of longer wavelength light. Thus, it is desirable for a broad spectrum emitting device to have a balanced distribution of unaggregated and aggregated dopants. Accordingly, Adamovich et al. describe alkyl derivatives of β-diketonates for use as single dopants that are less likely to aggregate and are more soluble due to the introduction of bulky alkyl groups.